1. Field of the Invention
The present invention relates to the field of the measurement of characteristics of a periodic digital signal and more specifically the measurement of the frequency and the jitter of a periodic digital signal with a digital measurement device.
2. Discussion of the Related Art
Many electronic circuits integrate periodic digital signal generation devices such as a phaselocked loop (PLL). Such periodic digital signal generation devices have to be tested to guarantee proper circuit operation.
In particular, it is necessary to measure the frequency and the jitter of the generated periodic digital signal to check that they are not outside guaranteed operating ranges.
Many devices enabling such a testing are available for sale. Conventionally, a dedicated time measurement unit (TMU) added to a conventional digital testing device is used.
FIG. 1 shows as an example the principle of the measurements performed by a TMU. Sin designates a periodic digital signal, the frequency and the jitter of which are desired to be measured, CK designates an internal clock signal of the TMU, t1 and t2 designate the contents of two counters incremented by clock CK, the counting of which is respectively triggered by a rising edge and a falling edge of signal Sin and is respectively reset by the falling and rising edges of signal Sin.
Once contents t1 and t2 are known, a calculator determines the frequency of signal Sin by calculating the inverse of t1+t2. Similarly, the TMU can perform a great number of measurements of value t2, and stores in a table the number of occurrences of each value measured for t2. Using this table a curve may be plotted representing the time distribution law of the rising edge of signal Sin, and a calculator deduces therefrom value a corresponding to the standard deviation of this distribution law, that is, the jitter of signal Sin.
A disadvantage of this method is that it requires a clock signal CK having a high frequency, generated by an expensive specific device. With technological developments resulting in a frequent use of devices such as PLLs generating periodic digital signals of increasing frequency, the state of the art will result in the use of TMUs operating at increasing frequencies and of an increasing cost.
An object of the present invention is to allow measuring the frequency and the jitter of a periodic digital signal without using a TMU.
Another object of the present invention is to provide a device allowing the measurement of the frequency and the jitter of a periodic digital signal, which can be integrated on the circuit which contains the periodic digital signal generation circuit.
To achieve these and other objects, the present invention provides a method for determining a characteristic of a periodic digital signal, including the steps of:
defining a measurement period such that the ratio between the measurement period and the period of the digital signal is a ratio of integers;
selecting a set of measurement periods in which the digital signal substantially has the same phase;
defining a measurement time having a same position in each measurement period of the set;
storing the value of the digital signal at each measurement time;
shifting the measurement time by a predetermined pitch smaller than one measurement period;
repeating the two preceding steps until the measurement time of each measurement period has scanned a predetermined portion of the measurement period; and
analyzing the succession of the stored values.
According to an embodiment of the present invention, the characteristic is the frequency of the digital signal;
the ratio between the measurement period and the period of the digital signal is an integer;
the predetermined portion of the measurement period is the entire measurement period; and
the analysis includes comparing the stored values with an expected value, storing in a table the number of inequations resulting from these comparisons for each position of a measurement time, and using the image of the digital signal provided by this table to calculate the period of the digital signal.
According to an embodiment of the present invention, the characteristic is the jitter of the digital signal;
the predetermined period portion is in a vicinity of an edge of the digital signal; and
the analysis includes comparing the noted values with an expected value, storing in a table the number of inequations resulting from these comparisons for each position of a measurement time, calculating the time derivative of the succession of numbers contained in this table, and using this derivative to calculate the jitter of the digital signal.
According to another aspect of the present invention, a method is provided for determining the frequency of a periodic digital signal, including the steps of defining a measurement period such that the ratio between the measurement period and the period of the digital signal is a non integer ratio of integers;
defining a measurement time having a same position in each period of a set of consecutive measurement periods;
storing the value of the digital signal at each measurement time; and performing on the stored values a reverse Fourier transform.
According to another aspect of the present invention, a device is provided for determining a characteristic of a periodic digital signal, including means for defining a measurement period such that the ratio between the measurement period and the period of the digital signal is a ratio of integers;
means for selecting a set of measurement periods in which the digital signal has substantially the same phase;
means for defining a measurement time having a same position in each measurement period of the set;
means for storing the value of the digital signal at each measurement time;
means for shifting the measurement time by a predetermined pitch smaller than one measurement period;
means for repeating the two preceding steps until the measurement time of each measurement period has scanned a predetermined portion of the measurement period; and
means for analyzing the succession of the stored values.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.